I tried running a community on it and it was a collosal failure. The onboarding flow sucks, if you want to send email logins it implicitly requires them to make matrix.org or whatever accounts (or something along those lines, details escape me), and you can have a custom server for that but it wasn't well documented and there was no canonical FOSS project for that custom server, I guess you were expected to just write your own if you wanted to truly control your whole stack.
And then, it was just high friction enough to where people wouldn't use it. Nobody downloaded the client apps other than me, even though the android one was really good, and even though you're spoiled for choice - you can even use it in Thunderbird! So everyone used the webapp, but then they'd switch computers and not do whatever you have to do to be able to read encrypted messages on the new machine, and so they'd lose all their messages and then stop participating.
And so on.
We moved the community to discord and all of our metrics have 10x'd: new users, existent user engagement, hell even revenue (we're an engineer-owned dev shop).
I really, really wish we could have made matrix work.